Thin planar optics are increasingly required for an array of applications, driven by the demand for smaller and smaller consumer products and semiconductor devices. This means optics manufacturers must take extra care to ensure that the glass they use is flat and free from deformities that can cause distortion and affect end-use functionality. This, in turn, calls for metrology tools that are able to accurately measure and confirm the uniformity of thin planar optics, thus proving they are fit for purpose.

Inherently, the measurement of thin parallel optics can be extremely taxing. Such optics are less than a few millimetres thick, meaning their front and back surfaces are very close together. It is therefore not possible to distinguish between the surfaces using the conventional mechanical measurement technique of phase shifting interferometry (PSI).

A more advanced technique is fourier-transform phase-shifting interferometry (FTPSI), since this enables easy characterisation of the front and back surfaces, optical thickness variation and material homogeneity of thin plane parallel glass. FTPSI makes it possible to distinguish between the front and back surfaces as well as characterise the quality of both in a single measurement, even if they are less than a millimetre thick.

Why fourier-transform phase-shifting interferometry?

To understand why FTPSI is the preferred technique for measuring thin parallel optics, it is necessary to take a closer look at the traditional PSI technique and see where it falls short.

PSI works by passing a light beam through an ideal reference optic (called a transmission flat (TF)) to the part under test. When properly aligned, the TF and the part under test create an interference pattern, recorded as an interferogram. The metrology software analyses the height variations produced by the phase shifts and reconstructs the surface wavefront, which represents the difference in height between the TF and the test part.

When the front surface of a thin parallel test part is aligned, a second reflection is typically returned to the interferometer from the back surface. This results in a complex fringe pattern created by multiple, overlapping interferograms, which is why it is not possible to distinguish between the front and back surfaces (see figure 1).

Figure 1

There are actions that can be taken to improve the situation, but they are time consuming and add unnecessary and potentially damaging steps to the overall process. These include applying black paint, marker ink or petroleum jelly to the back surface in order to eliminate its reflection.

The FTPSI technique eliminates the need to manually manipulate the back surface of a thin parallel test part to undertake meaningful and accurate measurements. Instead, it uses the reflection from the back surface to gain more information about the part in a single measurement.

The aforementioned is possible because FTPSI does not require mechanical motion within the test cavity to create the interferograms. Instead, it relies on modulation of the wavelength of the laser source to enable the measurement. Each cavity in the optical path in an FTPSI acquisition produces a unique interference frequency that defines its cavity length, which enables a clear delineation and accurate characterisation of the surface. Algorithms can then analyse both surfaces and characterise their form independently (see figure 2).

Figure 2b

In order to explain three- and four-surface FTPSI, it is necessary to begin with the basics. A TF, as mentioned above, is used with an interferometer to establish a plane reference for a surface or transmitted wavefront measurement. A reference flat (RF) is a high-quality optical surface that is used to direct a measurement beam with minimal effect on the overall wavefront.

The simplest FTPSI measurement is a three-surface configuration that consists of the TF (surface 1) and the test part (surfaces 2 and 3) (see figure 3). In this configuration, a back surface result is provided, but it includes material non-uniformities due to the measurement beam passing through the material of the test part.

Figure 3

Examples of a three-surface and four-surface configuration.

For higher accuracy measurements of the back surface, a four-surface configuration can be used by placing a RF (surface 4) behind the test part. In this configuration, the form of surface 3 is compared with the known RF. This configuration creates a second test cavity between the back surface and the RF and provides a direct measurement of the back surface without the uncertainty of the material in the part.

A single FTPSI measurement with both the three- and the four-surface cavity configurations includes a thickness deviation result, which is a full-surface map of the material thickness across the test part.

Material homogeneity

The four-surface cavity configuration described above enables the characterisation of the material homogeneity of the test part, a unique feature of FTPSI.

The homogeneity information can be obtained by first measuring the cavity with the test part inside, then removing the test part from the cavity and measuring the empty cavity, allowing a comparison between the TF and the RF. Unlike other homogeneity measurement techniques that only provide the nonlinear component, an FTPSI result maintains a fixed cavity and can therefore provide both the nonlinear and linear components of the material homogeneity. The linear portion is critical for applications that are sensitive to beam pointing, as the result can be used to predict how a beam deviates when passing through the test part.

Accuracy

As with all interferometric test methods, the measurement uncertainty is based on a number of factors including the quality of the reference optics, stability of the measurement environment and mounting techniques.

For parts less than 150 mm in diameter, the reference optic peak-to-valley surface form can be of the order of 2.5 percent of the wavelength of the light used to make the measurement—λ/40. For example, if the system has a laser emitting red light at a wavelength of 633 nm, this corresponds to approximately 16 nm. In most cases, this enables the resultant measurement to be well within the tolerance bandwidth for thin glass applications.

How the test part is held in the cavity is probably the most critical factor when measuring a thin optic, more specifically, the mounting technique and the mounting orientation. Simply clamping the optic can induce unwanted stress and cause it to bend. Differences in orientation can yield very different measurement results due to gravity effects. Ideally, the test part should be mounted in the same configuration in which it will be used in its end-use application to avoid unexpected differences between the designed intent and actual performance (see figure 4).

Figure 4a

Mounting examples for thin glass optics. The configuration in 4a is prone to overtightening, which can lead to distortion of the part, while the v-block configuration in 4b provides a low stress mounting solution.

Figure 4b

Mounting examples for thin glass optics. The configuration in 4a is prone to overtightening, which can lead to distortion of the part, while the v-block configuration in 4b provides a low stress mounting solution.

FTPSI is a compelling choice for optics manufacturers needing to ensure the quality of thin parallel optics. Unlike conventional mechanical PSI, FTPSI can distinguish between the front and back surfaces, and characterise their corresponding surface information in a single, repeatable measurement. Thanks to advances in both equipment and algorithms, FTPSI can characterise surface form, thickness deviation and material homogeneity of optics that are less than 1 mm thick. Its strengths in characterisation and ease-of-use make FTPSI an exceptional optical metrology solution for thin optics.